1. Field of the Invention
The present invention relates to a ceramic composite material which has a high mechanical strength and an excellent thermal stability of the microstructure in a wide range of temperatures from room temperature to a high temperature so that it can be suitably used as a structural material and a functional material exposed to a high temperature; it also relates to a porous ceramic material which has a high mechanical strength and an excellent thermal stability of the microstructure in a wide range of temperature from room temperatures to a high temperature so that it can be suitably used as a structural material and, a filter material, a reinforcing material for a metal or ceramics, a catalyst carrier, a thermal insulating material and other functional materials to be exposed to a high temperature.
2. Description of the Related Art
(I) SiC or Si.sub.3 N.sub.4 has been investigated to develop ceramic materials to be used at high temperatures but is not sufficient in high temperature properties. As an alternative material thereof, SiC/SiC composite materials produced by chemical vapor deposition, provided by Societe Europeene de Propulsion, have attracted attention and, at present are considered to be the best high temperature structural materials, and have been investigated and developed. The temperature range at which they can be used is reported to be 1400.degree. C. or lower.
A powder sintering method is the most popular method for producing ceramic materials. By improving the characteristics of powders, for example, by making the powder finer and purer, and controlling the conditions of sintering, a ZrO.sub.2 ceramic material having a strength as high as 3.0 GPa at room temperature has been produced. Further, in the powder sintering method, nano-dispersion of foreign ceramic particles in a ceramic composite material has been made possible and improvements in the strength, toughness, thermal conductivity, thermal shock resistance and other properties of the ceramic material are being sought.
Generally, it has been considered that oxide ceramic materials are not suitable for high temperature structural materials since they are easily deformed at a high temperature. However, oxide ceramic materials have oxidation resistance and corrosion resistance superior to any other ceramics. Accordingly, if the mechanical properties of the oxide ceramic materials could be improved, the oxide ceramic materials could be used in a wide range of applications as high temperature materials. In this respect, metal oxides having a melting point higher than 2000.degree. C., for example, Al.sub.2 O.sub.3, ZrO.sub.2, MgO, BaO, BeO, CaO, Cr.sub.2 O.sub.3 and rare earth element oxides such as Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Nd.sub.2 O.sub.3 and Er.sub.2 O.sub.3 are considered to be good candidates as high temperature ceramics.
Japanese Unexamined Patent Publication (Kokai) No. 5-85821, published in 1993, discloses a sintered body comprising a rare earth element oxide (or oxides of a mixture of rare earth elements) and Al.sub.2 O.sub.3 and a process for producing the same. In this process, a rare earth element oxide(s) and Al.sub.2 O.sub.3 are mixed and formed into a shape, which is then sintered at an appropriate sintering temperature and for an appropriate sintering time period so as to control the crystal grain size of the shape to be less than 30 .mu.m and prevent extraordinary grain growth and pores, to thereby provide a rare earth element oxide/alumina ceramic sintered body having a high strength and toughness which is reliable.
However, the room temperature strength may be significantly improved by controlling the production conditions and improving the starting powders, but the high temperature mechanical properties of ceramic composite materials are greatly influenced by the structure of the interface (grain boundary) between the particles of the component materials as well as by the interface (grain boundary) between and the crystal characteristics of the matrix and reinforcing phase. Further, at a high temperature, as the structure is finer, the more the superplastic property appears. Thus coexistence of the finer structure and the high temperature strength is difficult to attain.
Accordingly, a new method for producing a composite ceramic material which allows precise control of the above factors as well as a new composite ceramic material which has a novel structure and interface or grain boundary structure, are in demand.
The present inventors, considering the above problems of the prior art, have made investigation in order to provide a ceramic composite material which has unexpectedly improved high temperature properties.
As a result, the present inventors have invented novel composite materials, which consist of single crystal/single crystal, single crystal/polycrystal and polycrystal/polycrystal phases of an .alpha.-alumina phase and a YAG phase, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-149597 (published in 1995), 8-81257 (published in 1996) and 7-187893 (published in 1995).
The first purpose of the present invention is to provide, following the above composite materials consisting of an .alpha.-alumina phase and a YAG phase, novel ceramic composite materials, consisting of two or more crystals of different components or substances, the crystals having continuous three dimensional networks intertwined with each other, which composite material have excellent mechanical strength and structure thermal stability from room temperature to a high temperature, particularly greatly improved properties at a high temperature.
(II) In another aspect of the present invention, there is also provided novel porous ceramic materials, which are obtained from the above novel ceramic composite materials.
In comparison with metal and organic materials, the ceramics have such characteristics as (1) thermal characteristics of a high melting point, a small thermal expansion coefficient and a low thermal conduction, (2) mechanical properties of a high strength at high temperatures, a high hardness and a brittleness, and (3) electrical properties of a small electroconductivity, a large temperature dependency of electric resistance and a large dielectric constant. For example, Al.sub.2 O.sub.3 is chemically stable and hard and has a relatively high strength and an excellent electrical insulation and, therefore, it is widely used in various applications including insulating materials, abrasives, cutting tool materials, IC circuit boards, laser emitting materials, catalyst carriers and biomaterials.
Production of ceramics is currently being done mainly by the powder sintering method and, as a result, pores are necessarily included in the produced ceramics in grater or lesser degree. The powder sintering method is characterized in that the porosity, pore diameter and pore size distribution may be controlled by the control of the particle size and particle size distribution of starting powders and pressing pressure for shaping.
The porous ceramics having a certain level of porosity may exhibit a low density, a high specific surface area and other functions resulting from the penetrating pores, in addition to the characteristics inherent to the material of the ceramics. For example, a thermal insulator, an adsorber, a filter and so on.
Recently, vigorous investigation has been made to utilize ceramics having excellent thermal resistance, chemical resistance and mechanical properties by improving the functions thereof to an extreme level. Among them, ceramic filters, due to their excellent thermal, mechanical and chemical properties, are used as filters not only for filtering high temperature gases, chemicals, food and corrosive liquids but also for removal of metal oxides from molten metals and collection and removal of radioactive wastes.
Conventional methods for producing porous ceramics include (1) a process comprising forming a ceramic powder composition with an inorganic or organic binder, the particle size distribution of which is controlled, followed by sintering, (2) a process comprising mixing a ceramic powder with a pore-forming agent such as a polymer powder, an organic fiber and a carbon powder, forming the composition into a shape, and heating the shape to burn the combustible material and produce a ceramic with pores remaining therein, (3) a process comprising impregnating a foamed polymer body with a ceramic slurry, followed by heat treating it to burn the foamed polymer, (4) a process comprising heat treating a molten glass comprising two phases soluble and insoluble to a reagent at a temperature to form the two phases, followed by dissolving the soluble phase to form pores, and so on. However, in order to precisely control the pore size and distribution, provision of a porous crystal in which the pore is a part of the crystal, is required.
Recent diversity of the technology in various industrial fields makes applications and required properties of porous materials diverse. Particularly required properties of porous materials are uniform distribution of pores with a uniform pore size distribution, and a high porosity and high mechanical strength. However, satisfactory high porosity and high mechanical strength are difficult to obtain by the conventional methods.
For example, a conventional sintered silicon carbide consists of coarse plate crystals with less bonding between the crystals and, therefore, the mechanical strength is extremely low and the plate crystals may be peeled or the filter is fractured by shaking during handling. A porous ceramics produced by the sintering method comprises a glass flux or a clay material as a binder to bond the respective ceramic particles and, therefore, the ceramic material has low thermal and chemical resistances and a low flexural strength of not more than about 200 kg/cm.sup.2. Further, upon heating the binder is softened so that the porosity and pore size easily change and the mechanical strength may be significantly lowered as the bonding between the particles is weak. Moreover, a woven material and a felt- or block-like shape of a heat resistant inorganic fiber are used as a thermal insulator, a high temperature catalyst carrier, a filter, and a reinforcing member for various matrixes of resins, metals, glass and ceramics. However, since the bonding between the inorganic fibers is weak, the inorganic fibers may fly and disperse into the air (environment pollution problems), and since the mechanical strength is low, a honeycomb structure with a complex shape cannot be produced.
For example, Japanese Unexamined Patent Publication (Kokai) No. 3-232779 published in 1991 discloses a process for producing a porous silicon carbide sintered body excellent in an air permeability and mechanical properties. The starting silicon carbide particles used have a carbonaceous substance on the surface of the particles so that the grain growth of the silicon carbide is controlled to an appropriate level to form a porous body having a crystal structure in which relatively fine particulate crystals and relatively coarse plate-shape crystals co-exist and bonding between the crystals is increased. As a result, the mechanical strength is improved.
Japanese Unexamined Patent Publication (Kokai) No. 4-31375 published in 1992 discloses a process for producing a porous heat resistant member of a high purity mullite. A heat resistant inorganic fiber, an aluminum salt and a silicic acid sol are mixed and dispersed in water, and hydrolysis is conducted to form a co-precipitated sol of an aluminum silicate in the mixture. As a result, the heat resistant inorganic fiber and an aluminum silicate, which is highly active and excellent in sinterability, can be uniformly mixed and dispersed, so that a sheet-like shaped body with a high bonding between fibers can be obtained and the mechanical strength of the sheet is also improved. Therefore, it may be formed into a honeycomb.
Japanese Unexamined Patent Publication (Kokai) No. 4-65327 published in 1992 discloses a process for producing a high strength porous ceramics comprising a mullite whisker. It is described that the mullite whisker (1) contains less of gloss-forming alkali impurities, (2) is heat resistant and stable in shape in air up to about 1700.degree. C., and (3) has a relatively low thermal expansion coefficient. When the whisker crystals are three dimensionally intertwined to each other in sintering, continuous voids are formed in the sintered body. Using the mullite whisker or needle crystal, a ceramic material having a high strength and excellent pore-related properties such as the porosity and pore distribution can be produced by a simple process such as a process in which only the whisker is formed into a shape, or it is first mixed with a combustible substance such as a polymer or carbon powder followed by burning the combustible substance in air. Particularly, only a needle-like mullite crystal having a length of 1 to 20 .mu.m and a diameter of 0.1 to 3 .mu.m is formed into a pipe or a honeycomb by a mold pressing or other method, followed by heating in air at 1500 to 1700.degree. C. for 1 to 5 hours, by which it is mentioned that the porous property can be desirably controlled. Also, it is described that since the Al or Si component in the mullite inter-diffuses through the surface of respective crystals at 1500 to 1700.degree. C., sintering is accelerated and a sintering agent is unnecessary, so that the strength of the porous body increases. The porosity and the pore size can be desirable controlled by the size and amount of the mixed combustible substance. As a result, the obtained porous body had a porosity of 24.5% and the flexural strength of the porous body was as high as 980 kg/cm.sup.2 at 1700.degree. C. The properties of the porous body hardly changed after heating at 1000 to 1650.degree. C. for some tens of hours.
However, this means that the thermal stability of the porous body is only the level of some tens hours at 1650.degree. C. at the most.
Thus, in the sintering method, although the room temperature strength may be significantly improved by controlling the production conditions or selecting the starting powders, the mechanical property of a ceramics at a high temperature is greatly influenced by the structure of the crystal grain boundary and the crystallographic property of the constituting materials and the mechanical property of a ceramics at a high temperature is not stable or excellent as long as it is produced by the sintering method.
Therefore, a novel method for producing a porous ceramic material which allows precise control of the above-mentioned factors, and a porous ceramic material having such a novel microstructure resulting therefrom, are desired.
The purpose of this second aspect of the present invention is to provide a porous ceramic material which has both an excellent mechanical strength and an excellent thermal stability of the microstructure from room temperature to high temperatures, particularly these properties which are significantly improved at high temperature.